Non-contact methods of determining the position of an object based on generating a magnetic field and measuring its strength at the object are well known in the art. For example, U.S. Pat. No. 5,391,199, and PCT patent publication WO96/05768, which are incorporated herein by reference, describe such systems for determining the coordinates of a medical probe or catheter inside the body. These systems typically include one or more coils within the probe, generally adjacent to the distal end thereof, connected by wires to signal processing circuitry coupled to the proximal end of the probe.
U.S. Pat. No. 4,849,692, to Blood, describes a position tracking system based on detection of a DC magnetic field. Preferred embodiments described in this patent are based on detecting electrical currents generated in response to the field. Mention is made of the possibility of using a fiberoptic magnetic field sensor, but the patent gives no further information on possible implementations of such a sensor in position measurement.
The use of magneto-optic materials to measure magnetic field strength is known in the art, as described, for example, by M. N. Deeter et al., in "Novel Bulk Iron Garnets for Magneto-Optic Magnetic Field Sensing, Proceedings of SPIE, Vol. 2922, which is incorporated herein by reference. Magneto-optic materials rotate the polarization of polarized light passing through them, by an amount proportional to the strength of the magnetic field. The polarization rotation is characterized by a parameter known as Verdet's constant, expressed in units of deg/cm/Tesla. For strongly magneto-optic materials, such as yttrium iron garnet (YIG), the Verdet constant is about 10.sup.8. However, magneto-optic materials exhibit hysteresis, causing difficulties in field measurement when time-varying non-constant fields are involved.
Magnetostrictive fiberoptic strain gauges are also known in the art. For example, the article "Optical Fibre Magnetic Field Sensors," by K. P. Koo, Optics Letters, which is incorporated herein by reference, describes a method for measuring magnetic fields using magnetostrictive perturbation of a fiberoptic. A grating is produced within the fiber, for example by irradiating the fiber with an excimer laser. The grating generally comprises a periodically varying refractive index within the fiber. When light having a wavelength equal to twice the grating spacing is injected into the proximal end of the fiber, constructive interference of the reflected waves will give a strong reflection back to the proximal end. When a mechanical strain is applied to stretch the fiber, the grating spacing changes, so that the wavelength response of the reflected light is proportional to the mechanical strain and hence to the magnetic field.